Conventional lead-acid batteries such as the one depicted in FIG. 1a, comprise a series of separate (monopolar) positive and negative electrodes, connected in a combined series and parallel arrangement to achieve the voltage and current desired. Each electrode of this battery consists of a separate grid containing either the positive or negative active material which is suspended in an electrolyte. Cells are generally isolated from each other by partitions in the battery case. Insulating separators are used to keep the grids from touching.
The bipolar battery is fundamentally different from the conventional battery described above. The bipolar battery, depicted in FIG. 1b, is constructed of a series of bipolar battery plates, or biplates, which are solid sheets of material that have negative active material (NAM) on one side and positive active material (PAM) on the other side. The biplates partition the battery into cells and provide an electrical path between the NAM and PAM of adjacent cells. The biplates are separated by a separator, usually a glass mat material, in which the electrolyte is absorbed. The electrical current passes only through the thin biplates, perpendicular to the plane of the plates. This presents a very short distance and a very large cross-sectional area through which the current passes compared to the conventional lead-acid battery. Conventional batteries are characterized by a long electrical path between cells and a small electrical cross-section of the grid. As a result, the bipolar battery has almost no intra-cell resistance and about one-fifth the overall electrical resistance of conventional batteries. This reduction in resistance makes possible a high power battery that may be used to propel electric automobiles, provided that sufficiently long life, light weight and affordable construction cost can also be achieved.
Prior bipolar batteries have utilized biplates which are sealed at the edges by "edge seals", such as those illustrated in FIGS. 1c, 2a and 2b. These seals have three purposes:
1. Preventing the escape of water vapor and hydrogen or oxygen gas. This escape will cause the degradation of the battery due to loss of water from the electrolyte; PA0 2. Preventing liquid electrolyte from bridging between adjacent cells, thereby creating a leakage current between them resulting in self-discharge of cells; and PA0 3. Maintaining a predetermined spacing between biplates, which in turn, determines the amount of pressure, compression, or "crush" on the separator. PA0 1. The performance of the battery is dependent on obtaining and maintaining a very large number of gas-tight seals in each battery which has proved difficult to achieve in production; PA0 2. The assembly time of this type of seal contributes substantially to the overall cost of producing the battery; PA0 3. Although the edge seal can provide a means of physically spacing the bipolar plates, the optimum battery performance is achieved by maintaining a uniform separator pressure, not a uniform spacing, between the bipolar plate and the separator, which this type of seal does not allow; PA0 4. With this type of edge seal, shrinkage of the negative active material, the expansion of the positive active material and the relaxation of the separator cause dimensional changes which can not be compensated. Under these conditions, the compression force on the separator cannot be controlled; PA0 5. Forces at the joints resulting from separator compression, internal gas pressure, PAM expansion or NAM shrinkage continually "pry" apart and loosen this type of seal and thus allow leakage of electrolyte and gases; PA0 6. All joints must be gas tight for the life of the battery; because there may be more than 300 feet of joints in a battery, the reliability of the manufactured battery is poor; and PA0 7. In addition to maintaining the edge seals, each cell must have a vent for escape of gases and vapors during battery formation (initial charging) or abusive charging. Each cell must also be capable of being filled with electrolyte, which is cumbersome.
For decades, various methods of overcoming edge seal leakage have been attempted. These attempts have included the use of various adhesives and sealants, the "welding" of plastic joints or case materials around the plates, or the use of elastomeric seals under compression. Some of these approaches have resulted in outright failures. Some seals have been used to make serviceable batteries for a period of time. None, however, appear to have overcome the problems completely. A previous, rigid plastic edge seal is illustrated in FIG. 1c. This type of seal has these inherent problems:
Edge seals, such as those illustrated in FIG. 1c, 2a, and 2b, are made rigid, usually of plastic, to seal tightly against the gas vapor pressure in the cell. These kinds of seals hold the biplates apart and thus prevent a high clamping pressure from being applied to the separator. High clamping pressure applied to the separator has been shown to be advantageous to battery performance in conventional monopolar batteries.
Shrinkage of the negative active material or expansion of the positive active material during charging, and/or subsequent relaxation of the separator compression, also require the biplate spacing to change to maintain a constant compressive force against the glass mat separator. Expansion of active materials and/or grids within conventional lead-acid batteries can be a significant problem, because sufficient expansion can occur during the life of the battery which ruptures or splits the sides of the plastic battery case. In some prior bipolar batteries, maintaining the proper pressure on the separator and venting of the cell has been achieved by using a flexible elastomeric seal between the cells such as that illustrated in FIGS. 2a and 2b. This type of seal requires bipolar cell stacks to be manually adjusted during the fabrication process. Sometimes adjustment is necessary even after the battery is manufactured and is achieved by periodically tightening bolts holding the cell stack together. Without this adjustment, the loss of separator compression results in loss of battery performance. These tedious assembly steps are uneconomic and impractical for producing batteries at high rates.
The optimum compressive force on the separator, and thus the required clamping pressure on the cell stack that achieves the optimum battery life and power, has been found experimentally to range from about 7 to 20 pounds per square inch. Table 1 presents some experimental results showing the battery life and power benefits which can be achieved with a high and constant compressive force between biplate and separator.
TABLE 1 ______________________________________ "Standard" "High" % Compression Compression Increase ______________________________________ Life (cycles) 500-800 1635 100-227 Power (W/cm.sup.2) 1.68 1.84 10 ______________________________________
The data presented in Table 1 show that, in addition to controlling the spacing of biplates and accommodating any paste shrinkage and/or separator relaxation, the application of high pressure against the battery cell stack can impart a dramatic increase in battery life and a substantial increase in battery power.
The life-enhancing benefits of high separator pressure are the result of maintaining high pressure on the positive active material. The life of this electrode increases apparently when good particle-to-particle contact in the PAM is maintained. This can be seen indirectly by the increased life of batteries which use "tubular" rather than "flat plate" (grid type) positive plates. In the tubular positive plate, the active material is confined inside a strong, porous tube, usually made of woven fiberglass. Thus, as the material expands within a confined space, the particle-to-particle pressure continually increases, even to the point of rupturing the tube. Other attempts to increase battery life by confining materials expansion include various arrangements of clamps on the battery exterior including metal straps or "banding", and "cages" with plates and bolts. All these methods, however, rely on expansion of materials to generate and/or maintain an increased pressure on the PAM. The only method by which the pressure can be controlled and predicted, while accommodating all the variations of material expansion and contraction and gas pressure within a cell, is to apply a known and constant mechanical force. The benefits of high particle-to-particle pressure were shown directly in the results presented in a paper by J. Alzieu, B. Geoffrion, N. Lecause and J. Robert of Laboratoire deegnie Electrique des Universites Paris VI, at the Fifth International Electric Vehicle Symposium, Philadelphia, Pa., 2-5 Oct. 1978. This data showed that by applying a spring loading that was equivalent to fifteen pounds per square inch against the grids and separators of conventional monopolar batteries, a dramatic increase in battery cycle life could be achieved. However, the use of a spring-loaded cell stack in bipolar batteries has not been used previously because it requires an edge seal that offers little or no resistance to compression to transfer the compressive load to the separator, a condition at odds with using a rigid edge seal. Gas pressure within the cell can offset a significant portion of spring pressure. Also, the tensile loads on the battery case can become excessive for plastic materials which creep or slowly stretch, especially at higher temperatures. An electrode measuring seven inches by eight inches under a compression of fifteen pounds per square inch is subjected to an applied force of over 1000 pounds. This force must be carried as a tensile load through the thin plastic walls of a conventional battery case. If the plastic case stretches or "creeps", a constant force on the separator cannot be reliably maintained simply by pre-compressing the separator to a desired pressure. A positive means of applying a known pressure on the separator over a long period of time which allows for changing internal cell dimensions and gas pressure is required. One known means of applying pressure is an external mechanical arrangement for applying a spring force on the cell stack. Unfortunately, this usually increases battery size, weight and cost significantly. Applying the load externally to the battery as in the experiments of J. Alzieu, et al., and illustrated in FIG. 2 of their paper, adds impractical and unacceptable mechanical protuberances to the battery.
The use of springs within a battery is generally not viewed with favor by the battery industry because of the corrosion of the spring material and possible contamination of the electrolyte or negative electrode by metals, particularly iron. It is generally believed that acid "vapor" will reach the spring and quickly cause corrosion. Actual experiments in the construction of sealed bipolar lead-acid batteries using springs, however, show that only water vapor circulates within the battery. Little or no corrosion occurred in experiments lasting several months. Acid "creep" or wicking may eventually allow acid to reach the spring and thus initiate corrosion, however, spring coatings can prevent this.
There has frequently been concern in the battery industry regarding the use of commonly-connected cells wherein the vapors and gases are free to circulate between cells. This is partly due to the initial commercial failure of the "Torque-Starter.TM." battery, the first commonly-manifolded battery. The successful introduction of the "Optima.TM." battery, which uses commonly-manifolded cell, has demonstrated that commonly-connected cells are feasible.
The development of a practical method and apparatus which would provide the desired uniform, constant, high compressive force between the bipolar plates and separators in the cell stacks would constitute a major technological advance. The improved battery performance and longer useful life of batteries that could be obtained using such an innovative device would satisfy a long felt need within the battery industry in general and in the sealed lead acid battery industry in particular.